Resource-constrained devices are widely used in automation and control networks, for example, as battery-powered or even energy-harvesting controllers or sensors. Due to their limitations, for example, in terms of power, resulting in limitations in the available processing, reception and transmission time, the resource-constrained devices require a specific control solution to fulfill their role in the system.
There are dedicated protocols for handling such constrained devices, e.g. the solutions provided by the company EnOcean; there are also solutions becoming integrated into standard control technologies, such as the Green Power feature developed by ZigBee.
A resource-constrained device that has its radio inactive for substantial periods of time (such that no radio frame reception is possible during each period) is called a “sleepy” device. During such periods the device is said to be “sleeping” or “asleep”.
Typically, when a device sends a request to a resource-constrained device to obtain information, the message is not answered during the period in which the resource-constrained device is “sleeping”. Thus, the requesting device might have to send several requests before an answer with the requested information arrives at the requesting device. This is undesirable because it leads to a waste of processing and network resources and might even lead to congestion in the network via which the resource-constrained device communicates with other devices.
Published patent application US2012/0151028 provides a solution for this problem. In the network of the cited patent application, the “constrained devices”, which are devices that are in a “sleeping” mode on a regular basis, send their sleep and wake-up schedule to a non-constrained device in the network, which is, for example, an edge router of a low-power wireless network, and the non-constrained device may use the sleep and wake-up schedule for different purposes. For example, when a requesting device sends a message to the constrained device, the non-constrained device may temporarily cache the request and forward the request at the moment when the constrained device should be woken up according to the schedule. This particular solution has as a disadvantage that the requesting device still has to wait for an answer up to the moment in time that the constrained device wakes up.
Published patent application US2008/0120414 provides a solution in which resource-constrained devices provide their data values (e.g. values measured by their sensor) to an access point which is a non-constrained device and which is in contact with the resource-constrained device and another network (e.g. a wired network). The access point processes the received data values and stores the processed data. Another device may request the processed data from the access point. In this solution, the access point forms a sort of cache for data that is based on data that is provided by the resource-constrained device. The main goal of such a solution is that the resource-constrained device saves as much power as possible by entering the sleeping mode as much as possible, while another device is available to provide requested data as soon as possible. Unfortunately, this solution is not transparent for the resource-constrained device and the requesting device. Both devices need to know the address of the access point (cache) and, thus, they must learn the address of the access point. A further disadvantage is that the access point needs to be a powerful device which has a relatively large communication bandwidth; the access point is a centralized unit which receives data from many resource constrained devices and which has to answer requests from many requesting devices.